Ironically, the sensitivity of current detectors to signals from very large and massive astrophysical systems is limited by random noise in microscopic systems, that is, fluctuations resulting from the internal mechanical dissipation in the material constituting the mirrors used as test masses [1]}. In the most sensitive region of the detection band of Advanced LIGO [2]} and Advanced Virgo [3]}, between 50 and 300 Hz, the sensitivity is limited by an equal contribution of quantum noise [4]} and thermal noise, the latter being dominated by coating Brownian noise [5]}. Indeed, all GW detectors currently operating are laser interferometers with km-scale optical resonators [6]}, where the test masses used to probe the space time metric fluctuations are high-quality fused silica mirrors with high-reflection (HR) dielectric coatings on the front surfaces [7]}, [8]}, [9]}. Of all the sources of thermal noise in the mirrors, the Brownian motion of particles in the coating layers is the limiting factor and hence needs to be reduced for future sensitivity upgrades [10]}, [11]}, [12]}.
